1. Field of the Invention
The present invention relates to a power switching semiconductor device including an SI thyristor (static induction thyristor) and a MOSFET connected in cascade.
2. Description of the Prior Art
Cascaded BJT-MOS (bipolar-junction-transistor-MOS) semiconductor devices are known that have a low on state voltage and a high switching speed. They are used as power switching devices for motor drives or the like.
This device has a MOSFET 2 and a bipolar transistor 3 connected in cascade as shown in FIG. 1. The emitter 31 of the bipolar transistor 3 is connected to the drain 22 of the MOSFET 2 to form the cascaded arrangement. The base 32 of the bipolar transistor 3 is connected to a base voltage source 35. The semiconductor device is turned on by applying a voltage greater than a gate threshold voltage across the source 21 and gate 23 of the MOSFET 2. Specifically, when the MOSFET 2 conducts because of this, the transistor 3 is turned on because a forward bias voltage is applied across the base 32 and the emitter 31 of the transistor 3 from the base voltage source 35, and hence, a base current flows through the transistor 3 . This makes a collector current of the transistor 3 flow through the on-state MOSFET 2 . As a result, the entire semiconductor device is turned on, and the main current flows from an anode terminal 41 to a cathode terminal 42 .
On the other hand, when the applied voltage across the source 21 and the gate 23 of the MOSFET 2 is dropped to below the gate threshold voltage, the MOSFET is turned off. Hence, the base current of the bipolar transistor 3 is interrupted, and the main current flowing from the collector 33 to the emitter 31 of the transistor 3 changes its course to flow from the collector 33 to the base 32 when the MOSFET 2 is turned off. As a result, the junction between the collector 33 and the base 32 accomplishes rapid reverse recovery, and the entire semiconductor device is turned off quickly.
With this arrangement, the voltage in the off state is imposed on the junction between the collector 33 and the base 32 of the bipolar transistor 3. Consequently, the withstanding voltage of the device is determined by the V.sub.CBO of the transistor 3 rather than by the V.sub.CEO thereof which normally specifies the withstanding voltage of a transistor. As a result, the withstanding voltage of the transistor 3 becomes higher, and this makes it possible to employ a MOSFET with a low on state voltage and a low withstanding voltage as the MOSFET 2. Thus, high speed switching characterized by a low on state voltage is achieved.
FIG. 2 shows an example of the semiconductor device whose equivalent circuit is shown in FIG. 1. This semiconductor device is made on a single chip. In FIG. 2, like reference numerals attached to electrodes and terminals denote corresponding portions in the equivalent circuit shown in FIG. 1.
The semiconductor device comprises a bipolar transistor and a MOSFET which are connected in cascade. The bipolar transistor comprises a collector consisting of an n.sup.+ substrate 51 and an n.sup.- epitaxial layer 52 formed thereon, a base consisting of a p epitaxial layer 53 and an emitter consisting of n layers 54 selectively deposited on the layer 53. The MOSFET comprises a drain consisting of the n layers 54, a source consisting of n.sup.+ regions 56 selectively formed by a impurity diffusion process in a surface of p base layers 55 which are formed on the n layers 54, and gate electrodes 23 formed on the side walls of the n layers 54, the p base layers 55 and the n.sup.+ regions 56 via gate insulating films 57. Thus, the bipolar transistor and the MOSFET are connected in cascade through the n layers 54. In addition, an anode terminal 41, a cathode terminal 42, a base terminal 34 and a gate terminal 24 are provided. The anode terminal 41 is connected to a collector electrode 33 which makes contact with the n.sup.+ substrate 51; the cathode terminal 42 is connected to a source electrode 21 which makes a common contact with the exposed surface of the p base layer 55 and the n.sup.+ source regions 56, and is insulated from the gate electrodes 23 by interlayer insulating films 58; the base terminal 34 is connected to a base electrode 32 making contact with the p base layer 53; and the gate terminal 24 is connected to the gate electrodes 23.
The cascaded BJT-MOS semiconductor device described above is capable of high speed switching with a low on state voltage. In addition, since it is a gate driven device, its driving signal circuit becomes simple. The semiconductor device, however, has a problem in that it needs large driving power because of the base current fed to the bipolar transistor 3 in order to maintain the on state of the device.